It is important, in a geosynchronous communication spacecraft, to control the spacecraft's attitude in order to orient communication hardware relative to a planetary location. It is also important for momentum stored in a momentum wheel stabilization system of the spacecraft to be periodically unloaded or desaturated. Mechanisms to accomplish attitude correction and momentum desaturation ensure the geosynchronous spacecraft is kept on-station.
Several mechanisms are used by orbiting spacecraft for stationkeeping, attitude control and momentum desaturation. Typically, bi-propellant chemical thrusters are used to take a spacecraft from a transfer orbit to a synchronous orbit. The bi-propellant thrusters could also be used for stationkeeping, but such thrusters produce relatively powerful disturbance torques due to thruster misalignment, center of mass offsets, and plume impingements. As a result, the bi-propellant thrusters are not preferred for the fine adjustments which are typically made during stationkeeping maneuvers, attitude control, and momentum desaturation.
Alternatively, a spacecraft may use magnetic torquers for attitude control and momentum desaturation. In operation, magnetic torquers alter a spacecraft's attitude by applying the force of an electric coil's dipole moment with respect to the earth's magnetic field. However, magnetic torquers are not preferred for stationkeeping for they produce torque without lateral force. There are other disadvantages associated with the use of magnetic torquers in orbiting spacecraft. By example, magnetic torquers are heavy, provide little control power, and cannot be used to control the pitch attitude of the spacecraft due to the alignment of the dipole moment section with the magnetic field of the earth.
Conventional orbiting spacecraft may employ additional techniques for stationkeeping, attitude control, and momentum desaturation. The additional techniques include, for example, the use of ion propulsion thrusters. In fact, ion propulsion thrusters are particularly desirable for north-south stationkeeping on spacecraft with long mission lifetimes. However, a disadvantage of ion propulsion thrusters is their weight. Also, as with bi-propellant thrusters, the ion propulsion thrusters may produce disturbance torques on the spacecraft resulting in undesirable attitude motion.
It can be appreciated that in addition to the undesirable attitude motion caused by the conventional stationkeeping, attitude control and momentum desaturation techniques discussed above, undesirable attitude motion may also be caused by solar pressure imparted upon the orbiting spacecraft.
Momentum wheel stabilization systems are commonly used to counteract sources of disturbance torque. Typically, such stabilization systems include one or more momentum wheels and control loops to sense changes in the spacecraft's attitude. Sensors on the spacecraft may detect deviations on any of three orthogonal spacecraft axes; the x-axis (roll), the y-axis (pitch), or the z-axis (yaw). The control loops determine the required speed of the wheels to absorb pitch and yaw momentum based on the sensed attitude. Commonly, yaw and pitch momentum are absorbed directly by the momentum wheels, while roll momentum is absorbed as a change in yaw body angle which results from cross-coupling dynamics of momentum biased spacecraft. However, the roll momentum can be stored in the wheels if the wheel system design permits.
As was stated above, momentum stored in the momentum wheels must be periodically relieved, or desaturated. Desaturation is required in order to keep the momentum wheels within a predetermined speed range. Desaturation is typically accomplished by applying an external torque to the spacecraft through chemical propulsion thrusting or magnetic torquing. The external torque is applied to counteract, and thus reduce the stored momentum. However, chemical propulsion thrusting is not preferred for maintaining highly accurate spacecraft attitude during stationkeeping maneuvers because of the inherit disturbance torques introduced. Magnetic torques cannot be produced parallel to the magnetic field vector.
U.S. Pat. No. 5,349,532 Tilley et al. disclose a system and method to maintain a desired spacecraft attitude and unload accumulated momentum while performing north-south stationkeeping maneuvers. Tilley et al. disclose a device for sensing the spacecraft's position, attitude, and stored wheel momentum, and a device for determining the torques required to produce the desired attitude for the spacecraft and desaturate the momentum wheels. Tilley et al. teach in one embodiment the use of ion propulsion thrusters which are gimballed and throttled to produce the desired torques. Ideally, a spacecraft's stationkeeping and momentum management system would operate so as to maximize fuel efficiency during the mission lifetime, without sacrificing reliability. The inventors have determined that this goal would be realized with a system for simultaneously maintaining the spacecraft's stationkeeping maneuvers and momentum management, which employs a thruster firing cycle wherein thrusters are independently fired at predetermined locations along the spacecraft's orbit.